A magnesium alloy has come quickly into wide use as materials of a housing of a mobile-phone and a laptop computer or an automotive member because of its recyclability.
For these usages, the magnesium alloy is required to have a high strength and high toughness property. Thus, a producing method of a high strength and high toughness magnesium alloy has been studied in many ways from a material aspect and a manufacture aspect.
In a manufacture aspect, as a result of promoting nanocrystallizing, a rapid-solidified powder metallurgy method (a RS-P/M method) has been developed to obtain a magnesium alloy having a strength of about 400 MPa as much as about two times that of a casting material.
As a magnesium alloy, a Mg—Al based, a Mg—Al—Zn based, a Mg—Th—Zn based, a Mg—Th—Zn—Zr based, a Mg—Zn—Zr based, a Mg—Zn—Zr-RE (rare-earth element) based alloys are widely known. When a magnesium alloy having the aforesaid composition is produced by a casting method, a sufficient strength cannot be obtained. On the other hand, when a magnesium alloy having the aforesaid composition is produced by the RS-P/M method, a strength higher than that by the casting method can be obtained; however, the strength is still insufficient. Alternatively, the strength is sufficient while a toughness (a ductility) is insufficient. So, it is troublesome to use a magnesium alloy produced by the RS-P/M method for applications requiring a high strength and high toughness.
For a high strength and high toughness magnesium alloy, Mg—Zn-RE (rare-earth element) based alloys have been proposed (for instance, referring to Patent Literatures 1, 2 and 3).
Furthermore, an alloy containing Mg, 1 atomic % of Zn and 2 atomic % of Y, and an alloy containing Mg, 1 atomic % of Zn and 3 atomic % of Y, which is produced by a liquid quenching method, is disclosed in a Patent Literature 4. The alloy achieves to obtain a high strength property by making a fine-grained crystal structure by quenching.
Alternatively, a magnesium alloy, which is produced in such a manner that a casting product of an alloy containing Mg, 1 atomic % of Zn and 2 atomic % of Y is extruded at an extrusion rate of 4 and a temperature of 420° C. and then is subjected to a ECAE machining for 16 times, is disclosed in Non-Patent Literature 1. The idea of the Non-Patent Literature 1 is derived from the idea of the invention disclosed in Patent Literature 4, in which a fine-grained crystal structure is formed by quenching in order to obtain a high strength property. So, in this Non-Patent Literature, an ECAE machining for 16 times is carried out in order to form a fine-grained crystal structure.    Patent Literature 1: U.S. Pat. No. 3,238,516 (FIG. 1),    Patent Literature 2: U.S. Pat. No. 2,807,374,    Patent Literature 3: Japanese patent Application Laid Open 2002-256370 (Claims and Embodiments),    Patent Literature 4: WO02/066696 (PCT/JP01/00533),    Non Patent Literature 1: Material Transactions, Vol. 44, No. 4 (2003), pages 463 to 467.